Both dehydrogenation and aromatization of paraffinic feeds are highly endothermic reactions. Aromatization is believed to proceed via a two-step mechanism which first includes dehydrogenation and cracking of paraffins to form an olefinic intermediate and then dehydrocyclization of the olefinic intermediate to form aromatics. This mechanism was recognized by the teachings of U.S. Pat. No. 3,845,150 to Yan and Zahner which disclosed co-feeding a light olefinic supplemental feed stream together with a paraffinic stream to balance heat input to a catalytic aromatization zone. A second paraffinic stream may be dehydrogenated if a suitable light olefinic stream is not available.
Previous processes employing a single catalyst to promote paraffin aromatization have necessarily operated at less than optimal efficiency due predominately to two factors. First, relatively heavy coke deposition on the aromatization catalyst necessitated frequent regeneration and a relatively high rate of fresh catalyst make up. Second, previous processes were limited by inefficient heat transfer to the reaction zone.
Typical paraffin aromatization unit feedstreams readily form coke when raised to temperatures above about 450.degree. C. (1000.degree. F.) Coke precursors in the feed including oxygenates such as glycol readily react to form coke upon contact with the hot catalyst. This coke accumulates on the catalyst and blocks access to the active sites. Thus, both the catalytic activity as well as the aromatics selectivity are diminished as coke builds up on the catalyst.
Moreover, the relatively heavy coke loading on the catalyst increases heat input to the regenerator. This incremental heat input is disadvantageous as this additional heat must be removed to maintain the regenerator operating temperature. Excessively high regenerator temperatures can result in catalyst deactivation. The elevated regeneration temperatures usually associated with highly coked catalyst tend to accelerate steam deactivation. Steam deactivation, an irreversible physical degradation of the catalyst, is an integral function of time, temperature and water partial pressure. Therefore, the deposition of additional coke on the aromatization catalyst is clearly undesirable.
Previous processes for the fluid-bed aromatization of paraffins were still further limited by relatively inefficient heat transfer. Fresh feed was typically preheated in a process furnace to as high a temperature as could be sustained without excessive coke formation on the inside walls of the furnace tubes or other downstream facilities. Next, the fresh preheated feed was charged to the fluid-bed aromatization reactor where the hot fluidized catalyst brought the feedstream to reaction temperature almost immediately. Characteristics of the catalyst and the feed require that in this process configuration the bulk of the heat transferred to the feedstock be transferred by relatively inefficient indirect transfer. A more efficient method to heat the feed to reaction temperature would be by direct exchange with hot catalyst particles. At catalyst to feedstock ratios suitable for aromatization, however, the ratio of catalyst mass to feedstock mass and the heat input required by the strongly endothermic reaction would necessitate extreme catalyst temperatures. Raising the catalyst to such temperatures would likely cause a precipitous and permanent loss of activity.
From the foregoing, it can well be seen that direct heat transfer would be preferable over indirect due to its higher efficiency, particularly in a strongly endothermic fluid-bed reaction carried out at high temperature. Further, it is also clear that increasing the efficiency of the indirect heat transfer by raising the process furnace tube wall temperature would afford the desired increase in efficiency for only a brief period of time until the feedstock began insulating itself from the tube walls with a layer of coke, or until the accumulation of coke resulted in other even more detrimental operational problems. The most desirable heat transfer configuration would provide the efficiency of direct exchange while avoiding damage to the catalyst. Thus it would be highly beneficial to combine the advantages of direct heat transfer with those of a heat balanced reaction zone.